Offshore top site systems of this technical field are well known in the art. The floating unit is often a production platform or a vessel. Such floating units of offshore site systems are often referred to as FPSO units (floating production, storage and offloading units) or FSO units (floating storage and offloading units) depending on if the respective floating unit is adapted for processing hydrocarbons or not.
Such offshore top site systems often comprise a plurality of transportation lines extending between the floating unit and a subsea facility such as a well, a production equipment, a mid-water arch or a similar subsea facility. Such transportation lines are for example applied for transporting hydrocarbons and/or signals (via an optical fiber sensor) to the floating unit, water, CO2, drilling fluid, energy and other flowing media to the subsea facility. The transportation lines are often in the form of pipes or umbilicals.
For reducing the movements of the floating unit the floating unit will normally be moored using mooring lines, for example by mooring the floating unit to the seabed, to a tower or to one or more anchored buoys.
The transportation lines of such offshore top site systems are often rather heavy and provide a considerable pulling force at the point where they are connected to the floating unit.
Even though the floating unit is moored, weather conditions and tides provide high dynamic movements of the floating unit and the transportation lines including displacement motions (heave, sway, drift, and/or surge) and angular motions (yaw, pitch, and/or roll).
Offshore top site system comprising a floating unit (FPSO and FPO) has therefore mainly been used at shallow water applications, such as 300 m or less, where the transportation lines applied have been selected to be relatively small.
Further, such prior art offshore system has mainly been applied on sites with only minor currents and waves and often the offshore system has been equipped with relative expensive quick-release couplings to the transportation lines, which is adapted to be released in situation of increased currents and waves.
In order to apply offshore top site system comprising a floating unit (FPSO and FPO) at deeper water it has been attempted to alleviate the dynamic movements of the floating unit and the transportation lines by providing the floating unit with a turret system which alleviates some of the turning and rotational movements resulting from waves and winds. In situations where the transportation line is a steel pipe it is normally required that the floating unit should be equipped with a turret system.
Such turret systems are however rather expensive. Further it has been found that such turret systems do not to a sufficient degree alleviate the movements resulting from tides and vertical movements (heave) in general.
The connection from a transportation line to a floating unit is often rather stiff, e.g. provided by bolting a flange of an end of the transportation line to the floating unit. Such a connection will inevitably be subjected to very high and often damaging stress which leads to excessive fatigue and results in the need of replacement of expensive transportation lines.
WO 01/30646 describes a suspension device for a riser, for connecting the riser to piping on a fixed or floating platform such that the weight of the riser is transferred to the platform via an essentially moment-free hanger, thereby avoiding the use of quick-release couplings adapted to be released in situation of increased bending movements. The suspension device of WO 01/30646 has never been applied in practice.
US 2004/0057798 describes a steel catenary riser (SCR) system which is coupled to a vessel by a tensioning mechanism arranged on the floating vessel such as to ensure that a substantially constant tension is applied to the SCR such that the touchdown point of the SCR on the seabed is maintained stable. The SCR has an upper portion, which extends above the surface of the body of water and which is connected to the tensioning mechanism. This system is however only suitable at very shallow water since the control of the SCR is very limited and the risk collision with the vessel increases with the depth of the water due to the relative larger horizontal movement. For the same reason the described steel catenary system is not suitable for a system comprises several rises. Additionally it is generally desired to keep the number of moving mechanical facilities as low as possible, because the high forces such facilities will be subjected to require often repair or replacement, which results in temporary stop of production.
In situations where the transportation line is of the flexible type e.g. as described in standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008, it is often desired to apply a bend stiffener for protection of the flexible pipe against bend stress immediately adjacent to the connection to the floating unit. However, where the flexible pipe is relatively long, such about 1500 m or longer, experience has shown that the bend limiter is not sufficient to alleviate the stress resulting from vertical movements of the floating unit and the bend limiter almost merely has the function of moving the point of stress resulting from vertical movements to immediately below the bend limiter. Even though the bending moments immediately adjacent to the floating unit are relatively small where the water is deep and flexible pipe is long, the weight of the flexible pipe tends to result in undesired fatigue of the flexible pipe in the area where it is connected to the floating unit